I. Field of the Invention
This invention relates to transistor switching circuit arrangements and is most advantageously applicable to the switching on and off of switching transistors of high cut-off voltage.
II. Description of the Prior Art
The object of the invention is to provide improved transistor switching circuits which, while retaining the advantages of simple and inexpensive circuitry nevertheless achieve the desirable results of low power dissipation and fast switching. As will be later described, the invention provides improved, simple and inexpensive switching circuits in which low power dissipation is achieved by delaying the base current change to cut-off value which occurs on conversion into the blocked state and by a subsequent brief disruptive operating of the base-emitter of the switching transistor section, and fast switching is achieved by increasing the base current which occurs at the time of switching on.
On regulating a switching transistor having a relatively high cut-off voltage, for example a triple-diffused transistor, to change it into the blocked state from the conducting state, with strong base current values in the blocked direction, charge carriers present are withdrawn from the area of the base-emitter. If such a clearing operation of the charge carriers is performed rapidly by using a strong base current, then, because of the asymmetry of a switching transistor, the collector-base section continues to conduct for a brief time after this clearing of the base-emitter section. The reason for this is that the accelerated clearing of the base-emitter section produces a delay in the discharge of charge carriers.
Such slowed clearing of the collector-base section, however, involves an increase in the time taken to switch off the collector current, and this, in turn, produces an increase in the switching-off power dissipation of the transistor. It is possible to reduce this switching-off power dissipation by speeding up clearing of the collector-base section by delaying the change in base current to the cut-off current value at the time of cut-off. If, beyond this, the base-emitter section, after reaching the cut-off current value, is operated disruptively for a certain period of time subsequently, the clearing time of the collector base section can be further reduced. This is because any residual charges still present are discharged via the conducting base-emitter section. By resorting to both these expedients a considerable reduction of the switching-off power dissipation of the transistor is possible.
On the other hand, however, it is also desirable to have a fast switch on of the transistor, more especially if there is any substantial amount of inductance present in the regulating current circuit. In order to increase the speed of switch on of the transistor, the base current may be enhanced at the instant of switching on, thus reducing the delay in the increase of base current.
Known circuit arrangements capable of satisfying the foregoing requirements are very expensive and complicated. The reason for this is that they work on the principle of regulating the base by more than one control voltage, the control voltages used being without a common reference potential. In order to ensure the obtaining of correct values for these control voltages it is necessary to provide correspondingly potential-free monitoring circuits, more especially in cases in which current is being supplied to data processing apparatus. Such monitoring circuits are expensive and complex.